People protest the National Security Agency in Washington in 2014. REUTERS/Larry Downing By going over printing histories and work-computer data, the National Security Agency and the FBI traced a leaked report on the Russia investigation to 25-year-old Reality Leigh Winner.

According to the document, Russian military intelligence launched an attack on at least one US voting-software supplier and sent so-called spear-phishing emails to at least 100 local election officials shortly before the election. It is the strongest public indication so far that Russia interfered in the US election.

The affidavit says six people, including Winner, had printed the document. As a contractor for the NSA with Pluribus International, Winner had top security clearance.

According to the affidavit, after searching her work computer, NSA and FBI investigators discovered Winner had emailed The Intercept from a personal account and asked for a podcast transcript. Investigators also found that Winner searched the agency's computer system for the classified document on May 9 and printed it.

"Because the NSA logs all printing jobs on its printers, it can use this to match up precisely who printed the document," Robert Graham, a security expert, wrote for the blog Errata Security. The documents have small yellow dots that help track where they were printed, he said.

WikiLeaks accused the journalists at The Intercept of helping the NSA track Winner down so fast by handing over the printed documents.

On June 1, another government contractor told the NSA that a reporter had texted him pictures of the document to confirm whether it was real. The journalist said it was mailed from Augusta, Georgia.

Winner was arrested on Saturday at her home in Augusta. She has admitted to mailing the document and is in jail ahead of her trial.